A lambda regulation in connection with a catalytic converter is nowadays the most efficient exhaust gas purification procedure for the Otto engine. Only in combination with nowadays present ignition- and injection system very low exhaust gas values can be achieved.
Particularly efficient is the use of a three-way or selective catalytic converter. This catalytic converter has the feature to reduce hydrocarbons, carbon monoxide and nitrous oxides up to more than 98%, if the engine is operated in a range of about 1% around the air-fuel ratio with λ=1. The lambda value provides thereby how far the actually present air-fuel mixture deviates from the value λ=1, which is equivalent to a mass ratio of 14.7 kg air to 1 kg benzene that is theoretically necessary for a complete combustion, which means the lambda value is the quotient of the added air mass and the theoretical air demand. In the case of an air excess λ equals 1 (lean mixture). In the case a fuel excess λ is <1 (rich mixture).
During the lambda regulation each exhaust gas is basically measured and the added fuel amount and/or air amount is corrected correspondingly to the measuring result.
There are significant jumps in the lambda signal at the transition from boost operation into load operation at Otto engines, to which the suggested procedure mainly refers.
As sensors lambda probes are used, which can be constructed as a so-called two-point lambda probe or bistable probe on the one hand and as a continuous lambda probe or wide band lambda probe on the other hand. The effect of these lambda probes is based on the basically known principle of a galvanic oxygen concentration cell with a solid electrolyte. The characteristic line of a two-point lambda probe provides at λ=1 a jerky drop of the probe voltage. Therefore a two-point lambda probe, which is usually attached directly behind the exhaust manifold, allows basically only the distinction between rich and lean exhaust gas. A wideband lambda probe allows on the other hand the exact measurement of the lambda value in the exhaust gas in a wide range around λ=1. Both lambda probe types consist of a ceramic sensor element, a protection pipe, as well of cables, a plug and the connections between these elements. The protection pipe consists of one or several metal cylinders with openings. Through these openings exhaust gas enters by diffusion or convection and gets to the sensor element. The sensor elements of the two lambda probe types are thereby constructed differently.
A quick regulation of the exhaust gas composition on to the preset lambda value is significant for the low-emission operation of the combustion engine. This applies especially also for combustion engines with single cylinder regulation, at which the air-fuel mixture is adjusted individually for each single cylinder of the combustion engine on the basis of the signal of the common lambda probe. Therefore the lambda measurement has to take place with a high temporal resolution in order to be able to determine the consecutive exhaust gas volumes of the different cylinders in its composition that get to the lambda probe and to be able to assign them to a corresponding cylinder.
Besides the selected regulating parameters of the lambda control system and the distance parameters the dynamic of the lambda probe determines the speed of the control circuit. During restarting the dynamic of the lambda probes is thereby also sufficient for a single cylinder regulation with a common lambda probe in a common exhaust pipe for all cylinders. But due to ageing effects the dynamic characteristics of the lambda probes can change in such a way that the temporal resolution of the determination of the exhaust gas composition is not sufficient anymore, which causes an increased pollutant emission. If it lies outside the legal guidelines the lacking dynamic of the lambda probe has to be recognized in the range of the on-board diagnosis of the combustion engine and a corresponding error message has to be provided. In many countries the statutory provisions for motor vehicles require that such a diagnosis has to be implemented in the engine control unit, which turns on an error light at a slowing down of the lambda probes, which causes the exceeding of a default pollution threshold. In the USA the dynamic parameter that has to be monitored is précised as the so-called response-time, which means the time between a change of the oxygen or rich gas concentration in the exhaust gas at the probe and the corresponding change of the probe signal.
The state of the art knows a variety of diagnosing procedures, for example the comparison of the measured with an expected lambda signal at a known stimulation.
A procedure for diagnosing the dynamic characteristics of a lambda probe, which is used at least temporarily for a cylinder individual lambda regulation, as well as a corresponding diagnosing device are known for example from DE 102 60 721 A1. It is thereby provided that at least correcting variable of the lambda regulation is detected and compared to a default maximum threshold and is evaluated as not sufficient in the case of an exceeding of the maximum threshold of the dynamic behavior of the lambda robe with regard to the availability for the cylinder individual lambda regulation. The dynamic characteristics of the lambda probe can be detected from the single cylinder regulation itself because the cylinder individual regulators diverge at a not sufficient dynamic of the lambda probe. Furthermore a test function can be provided with a targeted interference or alienation of the actual lambda value. The procedure qualifies therefore only for combustion engines with a single cylinder lambda regulation or it requires a targeted influencing of the lambda value.
At present dynamic diagnoses usually single defined signal jumps are evaluated. An alternative procedure for diagnosing the dynamic of an exhaust gas probe provides that a simulated lambda value is calculated parallel to a lambda value that has been measured with the exhaust gas probe.
In order to be able to compare the calculated lambda value with the measured value also in dynamic driving operation, it is necessary to consider the gas travel time as well as the response behavior of the exhaust gas probe. A model exists therefore, which carries out a phase reverse rotation of the lambda value by a delay element of 1st order (PT1) and a dead time depending on the exhaust gas mass flow. The model parameters of this function are determined during the application and stored in the control unit. Thereby it can be ensured that the calculated and measured signals are in phase and therefore comparable.
This procedure requires a certain stability of the sensor behavior over lifetime. If the response behavior of the sensor changes, for example by depositing soot on the sensor element, the signal courses do not fit dynamically anymore. The result is that the application functions, which uses the simulated as well as the measured lambda signal, work with dynamically not matching input signals.
One application function is the so-called fuel mass observer (FMO), which is further described in a parallel application of the applicant. The fuel mass observer is a regulatory technical interference observer, which means an observer that is used for over-plugging disturbance variables. An observer is a model of the system that has to be regulated/controlled. This model distinguishes itself thereby that an output signal is compared to a measurement parameter of the real system. The difference between the simulated signal and the measured signal, the estimated error, are delivered back to the model inlet over a regulator. Thereby the model is regulated in such a way that the outlet behaves like the one of the real system.
Due to the above mentioned change of the response behavior big correcting variable deflections can occur during the FMO-output signal. The mixture is then for example made rich or lean at the wrong point of time. This has different effects from increased emissions up to component damages, for example due to increased exhaust gas temperatures at the turbo charger. This is only detected by a dynamic observation as it is already known from the state of art at an extreme change of the response behavior of the sensor. Only then the application functions can react upon it.
It is therefore the task of the invention to provide a procedure, which can detect in a deviation of the response behavior of the exhaust gas probe compared to the applied normal condition in the model behavior and which may correct it.